Bandstop filters are needed in many RF and microwave systems where they are used primarily to excise foreign interferers and mitigate co-site interference. In the case of wideband systems it essential that this filtering is achieved without sacrificing bandwidth, which requires that the bandstop filters possess wide passbands free of spurious responses. Unlike bandpass filters, where the upper stopband can be readily extended with the use of a lowpass filter, extending the passband of bandstop filters is a much more difficult problem.
The method typically used to extend the passband of a bandstop filter is to shift the higher-order resonances up in frequency with the use of stepped-impedance or lumped-element-loaded resonators. This approach has successfully been used to extend the pass and up to 6 times the fundamental frequency, e.g. as described in R. Levy, R. V. Snyder, and S. Shin, “Bandstop filters with extended upper passbands,” IEEE Trans. Microwave Theory Tech., vol. 54, pp. 2503-2515 (June 2006), but much beyond this the extreme physical dimensions of the resonators becomes a practical limitation.
It is therefore desirable to provide an approach that extends even further the upper passband to suppress the higher-order spurious resonances.